Filaments of Light

When shone through the atmosphere, a laser beam of sufficient
intensity does not spread out as it propagates. Rather, it travels
along a narrow filament, one that is perhaps 0.1 millimeter wide.
This curious phenomenon arises because the refractive index of air
depends on the intensity of the light, which is highest at the
center of the beam, causing it to come to a focus—as if it was
directed through a convex lens. This self-focusing effect continues
only up to a point: Eventually the intensity of the focused beam
becomes so high that electrons are stripped from the oxygen and
nitrogen atoms in the air. Such ionization would be expected to
cause the beam to diverge, but in combination with the self-focusing
that is also taking place, it allows light to travel for tens or
even hundreds of meters as a narrow filament. The author and his
colleagues have constructed a mobile terawatt laser that can create
such filaments of light, which can be used (among other things) to
probe the nature of aerosol particles.